1. Field of the Invention
The present invention relates to solid state imaging devices being manufactured in a CMOS- or MOS-technology. More particularly, a novel pixel structure leading to an improved image quality for the imaging devices is disclosed.
2. Description of the Related Art
Solid state image sensors are well known. Virtually all solid-state imaging sensors have as a key element a photosensitive element being a photoreceptor, a photo-diode, a photo-transistor, a CCD gate, or alike. Typically, the signal of such a photosensitive element is a current which is proportional to the amount of electromagnetic radiation (light) striking the photosensitive element.
A structure with a photosensitive element included in a circuit having accompanying electronics is called a pixel. Such pixel can be arranged in an array of pixels so as to build focal plane arrays.
Commonly such solid state image sensors are implemented in a CCD-technology or in a CMOS- or MOS-technology. Solid state image sensors find widespread use in devices such as camera systems. In this embodiment a matrix of pixels comprising light sensitive elements constitutes an image sensor, which is mounted in the camera system. The signal of the matrix is measured and multiplexed to a so-called video signal.
Of the image sensors implemented in a CMOS- or MOS-technology, CMOS or MOS image sensors with passive pixels and CMOS or MOS image sensors with active pixels are distinguished. An active pixel is configured with means integrated in the pixel to amplify the charge that is collected on the light sensitive element. Passive pixels do not have such means and require a charge sensitive amplifier that is not integrated in the pixel. For this reason, active pixel image sensors are potentially less sensitive to noise fluctuations than passive pixels. Due to the additional electronics in the active pixel, an active pixel image sensor may be equipped to execute more sophisticated functions, which can be advantageous for the performance of the camera system. The functions can include filtering, operation at higher speed or operation in more extreme illuminations conditions.
Examples of such imaging sensors are disclosed in EP-A-0739039, in EP-A-0632930 and in U.S. Pat. No. 5,608,204. The imaging devices based on the pixel structures as disclosed in these patent applications, however, are still subject to deficiencies in the image quality of the devices.
A first problem in these CMOS based imaging devices appears because material imperfections and technology variations have as effect that there is a nonuniformity in the response of the pixels in the array. This effect is caused by a nonuniformity or fixed pattern noise (FPN) or by a photoresponse nonuniformity (PRNU). Correction of the nonuniformity needs some type of calibration, e.g., by multiplying or adding/subtracting the pixel's signals with a correction amount that is pixel-dependent.
An example of such photoresponse nonuniformity correction method is disclosed in EP-A-0354106. The method shown in EP-A-0354106 is subtracting a current delivered by a current source from the signal acquired in the photosensitive element and only AC-currents are used in the further signal processing circuits.
A second problem in these CMOS based imaging devices appears because the pixel structures as disclosed in EP-A-0739039, EP-A-0632930 and U.S. Pat. No. 5,608,204 are sensitive to cross-talk on the photosensitive element of the pixels. This cross-talk arises from electronic components, for instance switches, in the amplifying circuits or amplifying parts of the pixels or being connected to the pixels. The pulses generated in such switches of the amplifying circuits or amplifying parts of the pixels can be of such magnitude that due to cross-talk of these pulses on the photosensitive elements of the pixels the image quality of the imaging devices based on this pixel can be significantly degraded. Specifically the requirement for a direct connection of amplifying transistor and photosensitive element in the pixel in EP-A-0632930 gives rise this problem.
Moreover the requirement for the short-circuiting of the gate and one of the electrodes (the drain in a p-MOS configuration) of the first transistor in EP-A-0632930, and the corresponding connection of the gate and the drain electrode to one fixed potential in order to achieve a logarithmic image conversion characteristic takes away design freedom in making such pixels and sensors. Specifically these latter requirements impede achieving other improved characteristics of the imaging devices than the logarithmic conversion characteristic of the imaging devices based on the pixel in EP-A-0632930.